1. Field of the Invention
This invention relates to a device for mixing optical signals. More specifically, the invention relates to an optical mixing device for detecting a received optical signal, suitable for an optical receiver used in a coherent optical communication system.
2. Description of Related Art
In an optical receiving apparatus used in a coherent optical communication system, a polarization diversity system may be effective to compensate for polarization fluctuations of an optical signal. Also, a balanced receiver system improves the sensitivity of the receiver. Therefore, an optical mixing device has applied a balanced polarization diversity system which is a combination of the two systems.
FIG. 7 shows a known optical mixing device. A polarization beam splitter module 71 includes a polarization beam splitter 711 which separates an input optical signal S from an information source (not shown) through an optical fiber 72a into two optical signals. One separated optical signal is supplied to a first optical directional coupler 73a through an optical fiber 72b. The other separated optical signal is supplied to a second optical directional coupler 73b through an optical fiber 72c. A local oscillating module 74 generates a local oscillating optical signal which is supplied to a third optical directional coupler 73c. Third optical directional coupler 73c divides the local oscillating optical signal into two optical signals which are applied to first and second adjusters 75a and 75b for adjusting the polarization planes of the optical signals. Local oscillating optical signals, with their polarization planes adjusted by adjusters 75a and 75b, are supplied to first and second optical directional couplers 73a and 73b through optical fibers 72d and 72e.
First optical directional coupler 73a mixes the optical signal from optical fiber 72b and the local oscillating optical signal from optical fiber 72d to supply mixed signals to a first optical detecting module 76a through optical fibers 72f and 72g. Second optical directional coupler 73b mixes the optical signal from optical fiber 72c and the local oscillating optical signal from optical fiber 72e to supply mixed signals to a second optical detecting module 76b through optical fibers 72h and 72i. First and second optical detecting modules 76a and 76b include twin detectors to convert received optical signals into electrical signals.
In the device shown in FIG. 7, polarization beam splitter module 71, local oscillating module 74 and first and second optical detecting modules 76a and 76b are connected optically through optical fibers 72a to 72i and adjusters 75a and 75b. A final adjustment of the device is executed by adjusters 75a and 75b. This final adjustment makes the device more difficult to employ.
FIG. 8 shows a known balanced polarization diversity receiver comprising an optical mixing device. The receiver includes a polarization beam splitter (PBS) module 81 which has lenses 811a to 811j and polarization beam splitters PBS1 to PBS3. An input optical signal Ps, which can be considered to have two perpendicularly polarized components with intensities aPs and (1-a)Ps, is supplied to PBS1 through an optical fiber 82a and lens 811a which causes the optical signals to form a parallel beam.
A local oscillating optical signal PL, which has two perpendicular polarized components with adjusted equal intensities PL/2, is supplied to polarization beam splitter PBS1 through a polarization-maintaining optical fiber 82b and lens 811b. Polarization beam splitter PBS1 combines perpendicular polarized components of signals Ps and PL to produce mixed optical signals. Each of the mixed optical signals, which has two perpendicular polarized optical signals, is focused to each of polarization-maintaining optical fibers 82c and 82d by each of lenses 811c and 811d. Each of polarization-maintaining optical fibers 82c and 82d is twisted by 45 degrees between opposite ends. Thus, each of the output optical signals from lenses 811e and 811f has a polarization plane rotated by 45 degrees to that of input optical signals of optical fibers 82c and 82d. Output optical signals from lenses 811e and 811f are supplied to polarization beam splitters PBS2 and PBS3.
Polarization beam splitter PBS2 receives a mixed optical signal consisting of components of signals Ps and PL that were in the perpendicular polarization planes and divides that mixed signal into two optical signals in two perpendicular polarization planes, each having components of signals Ps and PL. Polarization beam splitter PBS3 receives a mixed optical signal consisting of components of signals Ps and PL that were in the perpendicular polarization planes and divides that mixed signal into two optical signals in two perpendicular polarization planes, each of the optical signals having components of signals Ps and PL. The optical signals from beam splitter PBS2 and PBS3 are applied to optical fibers 82e to 82h through lenses 811g to 811j. When the mixing of optical signals Ps and PL causes PBS2 to produce an optical signal of large magnitude, which PBS2 introduces to one of optical fibers 82e and 82f, PBS2 will introduce an optical signal of low magnitude to the other of optical fibers 82e and 82f. This means that a balanced receiver can be formed by using output optical signals from optical fibers 82e and 82f. Therefore, output optical signals from optical fibers 82e and 82f are introduced to pin-photodiodes 841 and 842 in PIN-AMP module 84 through optical connectors 85a and 85b and optical fibers 82i and 82j. Electrical signals from photodiodes 841 and 842 are combined and amplified by amplifier 843 to be detected in delay detector 86. Optical fibers 82e, 82f, 82i and 82j are necessary for coupling PBS2 and pin-photodiodes 841 and 842.
Output optical signals from optical fibers 82g and 82h are processed in PIN-AMP module 85, to be detected in delay detector 87, through optical connectors 85c and 85d and optical fibers 82k and 821 in the same manner as PIN-AMP module 84. Optical fibers 82g, 82h, 82k and 821 are necessary for coupling PBS3 and pin-photodiodes 851 and 852. An adder 88 combines the two detected signals from delay detectors 86 and 87 to produce output signals independent of the polarization of input optical signal Ps.
The devices shown in FIGS. 7 and 8 employ many optical fibers which are unstable to changes in temperature. Also, highly accurate adjustments of the optical axes of the optical fibers are necessary for connecting the optical fibers. Nevertheless, an increase of coupling loss due to fiber coupling is unavoidable. Moreover, in the device shown in FIG. 7, the optical length difference between optical fibers 72f and 72g must be within 1 mm. Also, the optical length difference between optical fibers 72h and 72i must be within 1 mm. In the device shown in FIG. 8, the difference between the optical length from polarization beam splitter PBS2 to photodiode 841 and the optical length from polarization beam splitter PBS2 to photodiode 842 must be within 1 mm. Also, the difference between the optical length from polarization beam splitter PBS3 to photodiode 851 and the optical length from splitter PBS3 to photodiode 852 must be within 1 mm. Therefore, in the known devices, adjustments of the optical axes and optical lengths are troublesome. This deteriorates reliability. Furthermore, it is difficult to make the device compact because optical fibers cannot be sharply bent due to their material.